Autothermal reformers (“ATR”) are used to convert natural gas, steam and oxygen into synthesis gas (“syngas”) using a combination of partial oxidation and reforming. In gas-to-liquids (“GTL”) applications utilizing the Fischer-Tropsch process for the production of hydrocarbons, the preferred synthesis gas feed has an H2:CO ratio of between about 2:1 and about 2.2:1.
Commercial ATR systems currently in use for generating syngas for Fischer-Tropsch synthesis utilize O2 rather than air. Commercial ATRs employ a flame or ignition means and allow for the homogeneous partial oxidation reaction of natural gas, steam and air in a zone free of any catalytic material. The partial oxidation (“POX”) reaction creates hot gases which are typically in excess of 2200° F. and which then flow into a catalyst bed and undergo endothermic reforming while cooling. Relatively high, greater than about 0.6, steam to natural gas ratios must be employed in existing commercial ATRs in order to avoid soot formation within the high temperature region. Additionally, ignition means, such as burner nozzles and related mechanical equipments in existing commercial ATRs are complex and have limited operating life due to the stresses associated with high temperature operations.
Feed mixtures for existing commercial ATRs typically consist of air, steam and natural gas in ratios which result in an approximate 2.05 to 2.3H2:CO ratio. Such ATR feed gas ratios are typically in the following ranges:    Air/Natural Gas (A/NG) 2.5-3.2;    Steam/Natural Gas (S/NG) 0.6 to 2.0.
There are several factors that determine the specific ATR feed ratios appropriate for a particular application of the resulting syngas. Such factors include, but are not limited to, the composition of the natural gas, desired syngas compositions, and amount of molecular H2 added to the ATR feed mixture for hydrodesulfurization. The primary constituent of typical field natural gas is methane (>50 volume %) and the concentration of heavier hydrocarbon constituents, typically C2 to C10 hydrocarbons can range from about 1% to about 15%. Other non-hydrocarbon constituents, for example argon, nitrogen, CO2, and H2S, may also be present.
Existing commercial ATRs employ mixing of the Natural Gas, air and steam feed constituents. The NG and air are conveyed to the ATR separately and the steam may be fed into the ATR separately or alternatively, may be mixed with either the NG or air prior to feeding into the ATR.
In order to achieve the desired synthesis gas composition, existing commercial ATR operations generally occur at elevated temperatures in the range of 1600° F. to in excess of 2200° F. The design of any commercial ATR involves balancing several process variables including pressure, reactor volume and compression costs. In commercial ATRs utilizing an ignition means or flame, as the pressure increases the extent of methane conversion to CO diminishes. Moreover, higher pressures result in a higher volumetric heat release in the partial oxidation zone with the corresponding thermal, mechanical and soot formation issues.
In the startup of a commercial ATR system, initial ATR feed is typically an inert material, such as steam, nitrogen and possibly natural gas, with initial operation at temperatures less than 400° F. As the ATR temperature is increased, the ATR feed gas composition is transitioned to a mixture of steam and natural gas prior to the introduction of air or oxygen. Upon the introduction of air or oxygen and the transition to the ATR feed gas composition appropriate to producing a synthesis gas suitable, for example, for a Fischer-Tropsch process, the ATR feed gas mixture becomes flammable. A primary safety concern involves the introduction of flammable mixtures into process volumes downstream of the ATR. In ATR systems utilizing an ignition means or flame, the flammable ATR feed mixture undergoes partial oxidation in a specific volume within the reactor designed to handle the flow rates and temperatures associated with the combustion reaction. The ignition means or flame of commercial ATR ensures combustion of the flammable oxygen and natural gas mixture within the ATR and prevents the flammable mixture from exiting the ATR. In flameless ATR systems, however, there is a concern that all or part of a flammable feed mixture might not undergo POX reactions within the ATR and may flow into downstream components. Such failure to undergo POX might occur, for example, because of insufficient ATR catalyst activity. It is not desirable to permit the unreacted flammable feed mixture to exit the ATR because downstream equipment is not necessarily constructed to withstand the high temperatures/pressures generated in the POX reaction. To size and construct=the downstream equipment to safely incur such temperatures and pressures would be prohibitively expensive.
There exists a need for an ignition-less syngas production process which prevents introduction of flammable mixtures to process components downstream of the ATR. There further remains a need for a method to determine ATR catalyst activity.